Process for controlling a DC/DC converter with inductive storage and including an energetically neutral phase

ABSTRACT

A process for controlling a DC/DC voltage converter and a DC/DC voltage converter with inductive storage, operating according to cycles for transferring energy between a primary source and a secondary source. Each cycle includes a period of accumulation of magnetic energy in an inductive element from the primary source, followed by a period of restitution of this energy in the secondary source, wherein an energetically neutral phase of a duration such that the DC/DC voltage converter operates at a predetermined frequency is introduced into the operating cycles.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a process for controlling a DC/DCvoltage converter, or chopper, with inductive storage. Moreparticularly, the invention relates to such a process which, in a knownmanner, operates according to cycles for transferring energy between aprimary source and a secondary source, each cycle comprising a period ofaccumulation of magnetic energy in an inductive element from the primarysource, followed by a period of restitution of this energy in thesecondary source.

2. Discussion of the Background

DC/DC voltage converters with inductive storage comprise an inductivestorage element placed between a primary circuit with a controlledswitch interposed ahead of a power supply source and a secondary circuitwith switch interposed ahead of a load. The primary circuit ensures,through the tripping of its switch, the control of the cycles fordelivering energy from the power supply source to the inductive storageelement whilst the secondary circuit ensures the control of the cyclesfor delivering energy from the inductive storage element to the load.

Originally, these DC/DC voltage converters used a fixed choppingfrequency and possessed no regulation of their output voltage. The fixedchopping frequency was a beneficial characteristic when powering loadssensitive to radioelectric noise since the radioelectric noise of theconverter remained confined within a small band around its choppingfrequency and it was possible, by shifting this chopping frequency, tosucceed in minimizing the influence of the radioelectric noise of theconverter on its load by taking account of the latter's sensitivefrequencies. On the other hand, the absence of output voltage regulationwas very often a hindrance in so far as it was subject to sizeablefluctuations as a function of the variations in the power absorbed bythe load and of the variations in voltage of the power supply source.

At first, provision was made, at the output of DC/DC voltage converterswith inductive storage, for ballast-based voltage regulation in respectof loads sensitive to supply voltage variations. However, this resultedin a sizeable loss of efficiency. To solve this problem, DC/DC voltageconverters with inductive storage were subsequently equipped withservocontrols for regulating their output voltage which essentiallyalter the duration of their cycle of energy delivery to their inductivestorage element.

There are a large number of types of servocontrols for regulating outputvoltage for DC/DC converters with inductive storage. Among the mostsuccessful may be cited that described in French Patent ApplicationFR-A-2 729 516. These output voltage regulating servocontrols aregenerally satisfactory and improve efficiency to a great extent.However, they cause a variation in the chopping frequency as a functionof the power absorbed by the load and of the voltage of the power supplysource. This variation in chopping frequency produces a broadening ofthe noise band of the converter, rendering the problems withradioelectric spurious interference of the loads more difficult tosolve. Furthermore, it induces variations in the efficiency whichdecreases as the chopping frequency increases.

DC/DC voltage converters with inductive storage and output voltageservocontrol are well known, from the prior art, especially from GermanPatent DE-44 38 387, these converters operating at fixed frequency butposing problems both of efficiency and of radioelectric noise since theyengender large current pulses.

These converters comprise, as is customary, an inductive storageelement, a primary circuit with controlled switch interposed between apower supply source and the inductive storage element and a secondarycircuit with switch interposed between the inductive storage element anda load. Moreover, in their primary circuit, they exhibit an overcurrentdetector which causes the controlled switch to open in the event of anovercurrent, and in their secondary circuit they exhibit a device forshort-circuiting the terminals of the inductive storage elementtriggered as a function of the instantaneous voltage appearing acrossthe terminals of the load. Regulation of the voltage across theterminals of the load is obtained here by displacing the transitionbetween the periods of accumulation and of restitution of energy fromthe inductive storage element in the course of each chopping cycle ofconstant duration. To do this, a short circuit is created, in thesecondary circuit, earlier or later in each period of accumulation ofenergy in the inductive storage element, that is to say while thecontrolled switch of the primary circuit is conducting, this shortcircuit causing an overcurrent in the primary circuit and tripping theopening of the controlled switch by the overcurrent detector. Thisovercurrent pulse at each chopping cycle degrades the efficiency andincreases the radioelectric noise of the converter.

SUMMARY OF THE INVENTION

The present invention is aimed in particular at alleviating thesedrawbacks and, more especially at providing a DC/DC voltage converterwith inductive storage, and output voltage regulation, having, ascompared with those of the prior art, a lower level of radioelectricnoise, in a narrower frequency band, and better efficiency over agreater operating span, both as regards the voltage and the powerabsorbed by the load.

For this purpose, the subject of the invention is firstly a process forcontrolling a DC/DC voltage converter with inductive storage operatingaccording to cycles for transferring energy between a primary source anda secondary source, each cycle comprising: two separate phases, a phaseof accumulation of magnetic energy in an inductive storage element fromthe primary source and a phase of restitution of this energy in thesecondary source and comprising in order to do this, apart from the saidinductive storage element, a primary circuit with controlled switchinterposed between the primary source and the inductive storage element,and a secondary circuit with controlled switch interposed between theinductive storage element and the secondary source. In this controlprocess, an energetically neutral phase during which the inductivestorage element retains its energy, the controlled switches of theprimary and secondary circuits being open and means ensuring a zeroelectric voltage across the terminals of the inductive storage element,is introduced within a cycle for transferring energy, this energeticallyneutral phase having a duration within an energy transfer cycle suchthat the converter operates at a predetermined frequency.

This control process makes it possible to fix as is understood the widthof the span of variation of the chopping frequency of the converter,whilst allowing the implementation of the improvements described indocument FR-A-2 729 516. The advantages related to these improvementsare thus retained together with, furthermore, a new improvement in theefficiency obtained by lowering the magnetic losses and switchinglosses, the frequency being maintainable at a lower level, withoutincreasing the currents.

In particular, a constant operating frequency will generally be adopted,corresponding to that of the aforementioned document for the minimuminput voltage and maximum load conditions which achieve the bestefficiency.

Another advantage of operating at constant frequency is that it ispossible to synchronize the converter with other functions. It is thuspossible to reduce spurious interference, noise or frequency beating.

It is also possible, with a constant operating frequency, to constructinput filters tuned to this frequency.

Preferably, the said energetically neutral phase is started at a 0crossing of the voltage across the terminals of the magnetizinginductance of the inductive element.

Likewise preferably, the said energetically neutral phase is startedwhile the current passing through the magnetizing inductance is as smallas possible.

If this phase must start during a 0 crossing of the voltage, one in facthas the choice during the cycle between two instants. One will thereforechoose the one where the current is a minimum so as to minimize thecurrent flowing through the element ensuring a zero voltage across theterminals of the inductive element.

In one particular embodiment, the said inductive element isshort-circuited with the aid of an MOS transistor arranged in serieswith a diode, the closing of the transistor being commanded at aninstant at which the diode is reverse-biased.

The subject of the invention is also a DC/DC voltage converter withinductive storage, comprising an inductive storage element, a primarycircuit with controlled switch interposed between a primary source forproviding energy and the said inductive storage element, and a secondarycircuit with controlled switch interposed between the inductive storageelement and a secondary source for energy restitution and means forcyclically closing the controlled switches of the said primary andsecondary circuits during separate time intervals so as to endow theconverter with cyclic operation, with, in the course of each cycle, anenergy accumulation phase and an energy restitution phase, characterizedin that it comprises short-circuiting means for short-circuiting thesaid inductive storage element, and means for controlling the saidshort-circuiting means in such a way as to introduce into the operatingcycle of the converter an energetically neutral phase, during which theinductive storage element retains a constant energy, the controlledswitches of the primary and secondary circuits being open.

In a particular embodiment, the said inductive coupling elementcomprises a primary winding belonging to the said primary circuit, thesaid short-circuiting means comprising a controlled switch arranged inparallel with the said primary winding.

In another particular embodiment, the said inductive coupling elementcomprises a secondary winding belonging to the said secondary circuit,the said short-circuiting means comprising a controlled switch arrangedin parallel with the said secondary winding.

In yet another particular embodiment, the said inductive couplingelement comprises an auxiliary winding independent of the primary andsecondary circuits, the said short-circuiting means comprising acontrolled switch arranged in parallel with the said auxiliary winding.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is an electronic diagram of a converter according to a firstembodiment of the invention;

FIG. 1a is a modelling of the transformer represented in FIG. 1;

FIGS. 2-1 to 2-6 illustrate the six phases of the operating cycle of theconverter of FIG. 1;

FIG. 3 represents the manner in which the current in the magnetizinginductance and the voltage across the terminals of the primary of thetransformer change within the course of this cycle;

FIG. 4 is an energy diagram representing the manner in which the voltageacross the terminals of the magnetizing inductance and the currentpassing through it change in the course of the cycle; and

FIGS. 5 and 6 are diagrams similar to that of FIG. 1 of two otherembodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and moreparticularly to FIGS. 1-6 thereof, there are illustrated exemplaryembodiments of the present invention, as will now be described.

FIG. 1 depicts a DC/DC voltage converter comprising, in a known manner,a primary circuit 1 and a secondary circuit 2 which are coupled by atransformer 3. The primary circuit 1 comprises, in series with theprimary winding 4 of the transformer 3, a voltage source 5, here agenerator, and an MOS control transistor 6. Likewise, the secondarycircuit 2 comprises, in series with the secondary winding 7 of thetransformer 3, a voltage source 8, here a filtered load, and an MOScontrol transistor 9. A capacitor 10 is arranged in parallel with theprimary winding 7, likewise in a known manner.

FIG. 1a depicts the transformer 3 modelled as a perfect transformer withsolely the magnetizing inductance 11 of the transformer, of value Lmseen from the magnetizing medium. Np and Ns being the number of turns ofthe primary and secondary windings respectively, Ip and Is the currentin these windings, the magnetizing current has the value

ILm=NpIp+NsIs

The voltages across the terminals of the sources 5 and 8 will moreoverbe denoted Vin and Vout.

According to the invention, the primary circuit 1 furthermore comprises,in parallel with the primary winding 4, a diode 12 and an MOS transistor13 whose operation will be described hereinbelow.

This description of the operation of the converter of FIG. 1 will begiven with reference to FIGS. 2-1 to 2-6. The MOS transistors will, inthese figures, be modelled by their capacitors and spurious diodesduring the opening phases, and as short circuits during their closurephases. The spurious diodes will themselves be regarded as ideal, thatis to say as short circuits when they conduct, and as open circuits whenthey are disabled. Finally, the diode 12 is modelled as a capacitance inparallel with an ideal diode.

The first operating phase (FIG. 2-1), of duration Ton, is that of thestoring of energy in the transformer 3 from the source 5. The primaryMOS 6 is closed when the voltage at its terminals vanishes at the end ofthe previous phase, thus allowing smooth switching. The MOSs 9 and 13are open and the diode 12 conducts.

In the course of this first phase, the magnetizing current varies fromIon1m, negative, to Ion2m, positive. The voltage across the terminals ofthe magnetizing inductance remains constant at the value Vin/Np (FIG.4). The spurious diode of the MOS 6 conducts as long as the current Ipin the primary circuit is negative. The detection of a set-point valueIon2p causes the opening of the MOS 6, which starts phase 2, representedin FIG. 2-2.

During this second phase, of duration Tr, all the MOSs are open. Theopening of the MOS 6 causes a resonance between the primary inductanceLp and the capacitor Ceqp equivalent to all the capacitors of thecircuit taken back to the primary.

In the course of this phase, the energy is constant, and hence

ILm ² *Zeqm+VLm ²=Constant

and the operating point of the converter describes an arc of a circle inFIG. 4. The detection of the 0 v crossing of the voltage Vs across theterminals of the secondary winding causes the closure of the MOS 9 andthe start of the third phase.

This phase 3 is that of the restitution to the secondary of the energystored, during a time Toff. The MOS 6 is open and the MOS 13 is closed(the diode 12 is reverse-biased, so that the current in the MOS 13 iszero).

During this phase, the voltage VLm across the terminals of themagnetizing inductance is constant at the value −Vout/Ns (FIG. 4), andthe current ILm decreases from Ioff1m to Ioff2m (FIG. 3). The detectionof a set-point value at Ioff2s causes the opening of the MOS 9 and thestart of phase 4.

The opening of the MOS 9 brings about a resonance. The current ILscontinues to flow and charges the capacitance Cs of the circuit seenfrom the secondary. The MOS 6 is open and the MOS 13 closed.

In the course of this phase, the point of the diagram of FIG. 4describes an arc of a circle as in the course of phase 2, and thevoltage VLs increases in tandem with the charging of the capacitor bythe current ILs, until the diode 12 is turned on, thus interrupting theresonance and starting the fifth phase after a time Tf1 (FIG. 5).

In the course of this idle phase, the two MOSs 6 and 9 are open, and theMOS 13 closed. The diode 12 is on, imposing a zero voltage across theterminals of the primary winding 4. Thus, the transformer 3 keeps aconstant energy in the course of this phase, which is therefore neutralfrom the energy point of view.

The current ILm in the magnetizing inductance is constant, and thevoltage across its terminals is zero. The operating point on the diagramof FIG. 4 is fixed, on the vertical intensity axis. It will be observedthat this idle phase could have been triggered at the other zero voltagepoint of the diagram, but the short-circuit current would have beenhigher.

The duration Tv of this idle phase is adjusted so that

T=Ton+Tr+Toff+Tf1+Tv+Tf2

is constant and equal to the cutoff period, Tf2 being the duration ofthe sixth phase which will be described hereinbelow. It is this idlephase which makes it possible to operate at constant frequency.

When the duration Tv is reached, the MOS 13 is opened, by means notrepresented which are capable of calculating this duration, and theresonance phase resumes. All the MOSs are open. The operating point onthe diagram of FIG. 4 follows the arc of a circle of phase 4.

The detection of the 0 crossing of the voltage of the MOS 6 causes thestart of phase 1 of a new cycle.

It will be observed here that the closure of the MOS 13 is commanded byany suitable means at the same time as that of the MOS 9. This involvesa simple practical arrangement, it being possible for this closure to becommanded at any instant in phases 3 and 4.

FIGS. 5 and 6 illustrate two other embodiments, the manner of operationof which is equivalent to that of FIG. 1.

In FIG. 5, the MOS 15 for controlling the duration of the cycle and itsdiode 16 in series are connected in parallel with the winding 7 of thesecondary of the transformer, so as to short-circuit it during the idlephase.

In FIG. 6, the transformer comprises a third winding 17. The MOStransistor 18 for controlling the duration of the cycle and its diode 19are here connected to this winding so as to short-circuit it during theidle phase.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:
 1. A DC/DC voltage converter, comprising: aninductive storage element; a primary controlled switch between a primarysource and said inductive storage element; and a secondary controlledswitch between said inductive storage element and a secondary source;wherein said DC/DC voltage converter is configured to operate accordingto a cycle for transferring energy between a primary source and asecondary source, said cycle includes a phase of accumulation ofmagnetic energy in said inductive storage element from said primarysource and a phase of restitution of the accumulated magnetic energy insaid secondary source, said DC/DC voltage converter is configured toinclude an energetically neutral phase in said cycle during which saidinductive storage element retains energy therein, said primary andsecondary controlled switches are opened, and a zero voltage ismaintained across terminals of said inductive storage element, and saidDC/DC voltage converter is configured such that said energeticallyneutral phase has a duration within the energy transfer cycle such thatsaid DC/DC voltage converter operates at a predetermined frequency. 2.The converter of claim 1, further comprising configuring said DC/DCvoltage converter such that said predetermined frequency is an operatingfrequency at minimum voltage and maximum load.
 3. The converter of claim1, wherein said DC/DC voltage converter is configured such that saidenergetically neutral phase is started at a zero crossing of a voltageacross said terminals of a magnetizing inductance of said inductivestorage element.
 4. The converter of claim 3, wherein said DC/DC voltageconverter is configured such that said energetically neutral phase isstarted while a current passing through said magnetizing inductance isat substantially a lowest value.
 5. The converter of claim 1, whereinsaid DC/DC voltage converter is configured to short-circuit saidinductive storage element to provide said energetically neutral phase.6. The converter of claim 5, wherein said DC/DC voltage converter isconfigured to short-circuit said inductive storage element to providesaid energetically neutral phase by short-circuiting a primary windingof said inductive storage element via a MOS transistor arranged inseries with a diode, and said DC/DC voltage converter is configured toclose said transistor while said diode is reverse-biased.
 7. Theconverter of claim 5, wherein said DC/DC voltage converter is configuredto short-circuit said inductive storage element to provide saidenergetically neutral phase by short-circuiting a secondary winding ofsaid inductive storage element via a MOS transistor arranged in serieswith a diode, and said DC/DC voltage converter is configured to closesaid transistor while said diode is reverse-biased.
 8. The converter ofclaim 5, wherein said DC/DC voltage converter is configured toshort-circuit said inductive storage element to provide saidenergetically neutral phase by short-circuiting an auxiliary winding ofsaid inductive storage element via a MOS transistor arranged in serieswith a diode, and said DC/DC voltage converter is configured to closesaid transistor while said diode is reverse-biased.
 9. A process forcontrolling a DC/DC voltage converter having an inductive storageelement, comprising the steps of: operating said DC/DC voltage converteraccording to a cycle for transferring energy between a primary sourceand a secondary source; configuring said cycle to include a phase ofaccumulation of magnetic energy in said inductive storage element fromsaid primary source and a phase of restitution of the accumulatedmagnetic energy in said secondary source; interposing a primarycontrolled switch between said primary source and said inductive storageelement; interposing a secondary controlled switch between saidinductive storage element and said secondary source; providing anenergetically neutral phase in said cycle during which said inductivestorage element retains energy therein, said primary and secondarycontrolled switches are opened, and a zero voltage is maintained acrossterminals of said inductive storage element; and configuring saidenergetically neutral phase to have a duration within the energytransfer cycle such that said DC/DC voltage converter operates at apredetermined frequency.
 10. The process of claim 9, wherein said stepof providing said energetically neutral phase comprises starting saidenergetically neutral phase at a zero crossing of a voltage across saidterminals of a magnetizing inductance of said inductive storage element.11. The process of claim 10, wherein said step of starting saidenergetically neutral phase comprises starting said energeticallyneutral phase while a current passing through said magnetizinginductance is at substantially a lowest value.
 12. The process of claim9, further comprising configuring said predetermined frequency as anoperating frequency at minimum voltage and maximum load.
 13. The processof claim 9, wherein said step of providing said energetically neutralphase comprises short-circuiting said inductive storage element.
 14. Theprocess of claim 13, wherein said step of short-circuiting saidinductive storage element, comprises: short-circuiting an auxiliarywinding of said inductive storage element via a MOS transistor arrangedin series with a diode; and closing said transistor while said diode isreverse-biased.
 15. The process of claim 13, wherein said step ofshort-circuiting said inductive storage element, comprises:short-circuiting a secondary winding of said inductive storage elementvia a MOS transistor arranged in series with a diode; and closing saidtransistor while said diode is reverse-biased.
 16. The process of claim13, wherein said step of short-circuiting said inductive storageelement, comprises: short-circuiting a primary winding of said inductivestorage element via a MOS transistor arranged in series with a diode;and closing said transistor while said diode is reverse-biased.